Elementary electrochemical cells used to separate oxygen from air, or from a gas mixture containing it, are generally formed from a ternary system consisting of solid electrolyte/electrodes/current collectors.
The solid electrolytes used for separating oxygen from a gas mixture are doped ceramic oxides, which, at the operating temperature, are in the form of a crystal lattice having oxide ion vacancies. The associated crystal structures may, for example, be fluorite, perovskite or brown-millerite cubic phases called aurivillius phases; J. C. Boivin and G. Mairesse have referenced all the O2− anionic conducting crystal phases in a general article (Chem. Mat., 1998, p. 2870-2888, “Recent Material Developments in Fast Oxide Ion Conductors”).
The electrode materials associated with the solid electrolyte are generally perovskites. These are materials possessing a crystal structure of the ABO3 or AA′BB′O6 type (A, A′: lanthanide and/or actinide; B, B′: transition metals) based on the structure of natural perovskite CaTiO3. These materials exhibit good hybrid (ionic/electronic) conductivity properties thanks to this cubic crystal structure, in which the metal ions lie at the corners and at the centre of an elementary cube and the oxygen ions at the middle of the edges of this cube. The electrode materials may also be perovskite material/purely ionic conducting material mixtures or else mixtures based on materials possessing other crystal phases, for example of the aurivillius, brown-millerite or pyrochlore type.
Current is collected either by a metal or a metal lacquer or by a metal/“inert oxide” (such as alumina) ceramic mixture, or by a metal/carbide (such as silicon carbide) mixture or by a metal/nitride (such as silicon nitride) mixture, in which the principle role of the oxide, carbide or nitride is that of mechanically blocking the segregation/sintering phenomena that appear owing to the high operating temperatures (700° C.<T<900° C.), especially when silver is used as current collector metal, or by a metal/“hybrid conductor” oxide ceramic (such as an oxide of the perovskite structure of the family of strontium-doped lanthanum manganites) mixture or by a metal/“ionic conductor” oxide ceramic (such as yttrium-stabilized zirconia) mixture.
However, the Applicant has found that when a tubular electrochemical cell closed at one end, in which the solid electrolyte is zirconium oxide stabilized with 8% (molar) yttrium oxide (8 mol % YSZ), the electrodes are made of La0.9Sr0.1MnO3-δ (LSM), the cathode current collector is a silver lacquer and the anode current collector is a gold lacquer, is operated at a temperature of between 700 and 1000° C., whether at atmospheric pressure or under an internal oxygen pressure of between 1 and 50×105 Pa (1-50 bar) or under an external oxygen pressure of between 100 and 150×105 Pa (100-150 bar), a high cell potential (around 1.7-1.8 V) for a low applied current (3-3.5 A, i.e. 0.03-0.04 A/cm2) is obtained. As a comparison, units having silver lacquer as anode current collector have, for maximum working pressures of 10 to 20 bar of oxygen, cell potentials of around 1 to 1.5 V for current densities of 0.15 A/cm2.
For tubular cells made of yttrium-doped zirconia with silver-based anode current collectors and at oxygen pressures of greater than 50 bar and temperatures above 750° C., a phenomenon is observed in which the said metal evaporates because of the lowering of the melting point Tm(° C.) of this metal by the dissolution of oxygen, according to the equation:Tm(Ag)=961−22.31p1/2 The consequence of this evaporation is a rapid degradation in the electrochemical performance of the cells, characterized by a sudden increase in the potential and a drop in the coulombic efficiency. In the case of oxygen production at high pressure (>50×105 Pa) by a YSZ tubular cell, the use of silver is therefore to be proscribed because of its physico-chemical properties. This is why the present silver-based current collector, in lacquer form or in the form of a silver/ceramic oxide cermet, is generally replaced with an anode current collector made of gold lacquer.
This substitution allows the cells to operate at high pressure (p(O2) between 50 and 150×105 Pa), but their electrochemical performance remains poor. It is necessary to maintain a low productivity (applied current per tube of around 3 A) in order to ensure that the cell is stable over time. This is because very rapid degradation in the potential of the cell is observed when it is attempted to remain at a satisfactory, namely higher, productivity level (5 to 7 A i.e. 0.05 to 0.07 A/cm2).
The Applicant started from the assumption that, as a result of the observations of cells after operation, the low productivity observed (applied current density of less than 50 mA/cm2 for temperatures above 800° C.) and the degradation of the cell potential, observed if the productivity were to be increased by a factor of 1.5, are the consequences of an unsuitable architecture of the cell used.
The term “architecture” is understood to mean the structures and microstructures of the various constituent materials of the ceramic membrane, namely the solid electrolyte (8 mol % YSZ, yttrium-stabilized zirconia), the electrode (LSM: strontium-doped lanthanum manganite) and the current collector (silver lacquer or silver/oxide or non-oxide ceramic cermet on the cathode side; gold lacquer on the anode side). The term “structure” is understood to mean the chosen system of stacked layers and the order of the various coatings deposited in order to make up an electrochemical cell (solid electrolyte/electrode/current collector) and the geometrical shapes (tube, plate) of the membranes.
The term “microstructure” is understood to mean the thicknesses, densities, areas and roughness within the various materials characterizing the membrane, the sizes and morphologies of the grains and/or particles of the various materials, the intergranular and intragranular porosity of the solid electrolyte the nature (morphology) of the surface of the solid electrolyte, the porosity and staking of particles of the various coatings electrode, current collector.
The Applicant assumed that the absence of porosity in the gold-based current collector could limit the diffusion and/or dissolution in this layer of “recombined” gaseous oxygen on the anode side and could result in a high overvoltage and consequently a low productivity.
In addition, the Applicant also observed that, when the cells used hitherto operated with a higher current density, so as to achieve a higher productivity (0.05-0.07 A/cm2 as opposed to 0.03-0.04 A/cm2), the electrode/gold-lacquer-based anode current collector coatings debonded from the external surface of the membrane (anode side: oxygen production, high pressure >100 bar) but also there was debonding of the electrode/solid electrolyte interfaces.
It also assumed that, since the increase in the productivity results in greater evolution of oxygen gas, this gas could not be rapidly removed because of the absence of porosity in the anode current collector layer was therefore one of the causes of this debonding, which could in addition be favoured by the low adhesion forces and the weak interactions between the solid electrolyte, the LSM electrode layers and the gold layer because of the low sintering temperature (<850° C.), required by the presence of silver on the cathode side, and because of the cosintering of the unit during its manufacture.